Apparatus for an electron gun employing a thermionic electron source

ABSTRACT

An apparatus for an electron gun includes a one piece thermionic electron source that extends in a longitudinal direction and has opposite end portions, an aperture disposed therebetween, and two longitudinal portions that extend in the longitudinal direction and are spaced apart from one another by the aperture and rigidly joined together by the opposite end portions. The thermionic electron source is less susceptible to ion bombardment and therefore has a longer operating life than previous ribbon type electron sources. Further, because the electron source is one piece, it does not require support and relative positioning of multiple emitters such as that required by an electron source having two separate and parallel emitters.

RELATED APPLICATION

The subject matter herein may be disclosed and/or claimed in U.S. patentapplication Ser. No. 09/210,632 entitled “METHOD AND APPARATUS FOR USEWITH AN ELECTRON GUN EMPLOYING A THERMIONIC SOURCE OF ELECTRONS”.

TECHNICAL FIELD

This invention relates to a thermionic source of electrons for anelectron gun, and more particularly to a one piece thermionic source ofelectrons with an aperture.

BACKGROUND

Electron guns are used to heat materials to produce vapors of thematerials for deposition on an article. The electron gun typicallyincludes an electron source, a focusing electrode, and an acceleratingelectrode. The electron source is typically a cathode heated by anelectric current to cause the cathode to emit electrons. The focusingelectrode is typically negatively charged to repel the electrons andthereby direct the electrons in a direction generally toward theaccelerating electrode. The accelerating electrode is positioneddownstream from the electron source and the focusing electrode. Theaccelerating electrode is typically less negatively charged than theelectron source and the focusing electrode to cause the electrons toform into a beam and travel in the downstream direction.

One type of thermionic electron gun has a single elongated electronsource having a ribbon shape. The ribbon shaped electrode is desirablefor its simplicity and ease of use. However, many ions are produced as aresult of the vaporization of the materials. Positively charged ions maybe attracted to the negatively charged focusing electrode and therebybombard and erode the electron source. The erosion has the effect ofdegrading the performance of the electron source and shortening itsuseful life.

U.S. Pat. No. 3,701,915 discloses an electron source that is lesssusceptible to bombardment by ions. The electron source is made up oftwo elongated emitters spaced apart and parallel to one another. Becauseof the focusing tendency of the focusing electrode, the ions do notstrike the emitters but rather pass through the space between them andhit the focusing electrode. A substantial improvement in the life of theemitters is achieved. However, a two part electron source cannot be usedon a gun designed for a single electron source unless the gun ismodified. For example, a two part electron source requires means toelectrically connect to and support the two emitters, as well as meansto properly space the two emitters apart.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved linearthermionic electron source that has a longer operating life thanprevious ribbon type linear thermionic electron sources yet does notrequire a two part electron source.

The present invention is predicated in part on the recognition that aribbon type electron source can be one piece yet made less susceptibleto ion bombardment and therefore longer operating, by providing anaperture in the electron source whereby ions do not strike the electronsource but rather pass through the aperture.

According to the present invention, an apparatus for an electron gunincludes a one piece thermionic electron source that extends in alongitudinal direction and has opposite end portions, an aperturedisposed therebetween, and two longitudinal portions that extend in thelongitudinal direction, are spaced apart from one another by theaperture and rigidly joined together by the opposite end portions.

In accordance with one detailed aspect of the invention, each of theemitter portions has a surface that is inclined toward that of theother.

The present invention provides an improved linear thermionic electronsource that is less susceptible to ion bombardment and therefore has alonger operating life than previous ribbon type electron sources, yetdoes not have many of the disadvantages of an electron source with adual emitter. Because the improved electron source is one piece, thesource does not require support and relative positioning of multipleemitters such as that required by an electron source having two separateand parallel emitters. In addition, the one piece construction may makethe electron source more rigid and thus more durable and less likely todeform than an electron source having two separate emitters.

These and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description, accompanying drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective, partially exploded, partially cut away view ofan electron gun;

FIG. 2 is a perspective view of a thermionic electron source inaccordance with one embodiment of the present invention for use in theelectron gun of FIG. 1;

FIG. 3 is a cross section view in the direction of 3—3 of FIG. 2, of thethermionic electron source of FIG. 2;

FIG. 4 is a cross section view in the direction of 4—4 of FIG. 2, of thethermionic electron source of FIG. 2;

FIG. 5 is a cross section view in the direction of 5—5 of FIG. 2, of thethermionic electron source of FIG. 2;

FIG. 6 is a cross section view in the direction of 6—6 of FIG. 1, of thefocusing electrode and the thermionic electron source used in theelectron gun of FIG. 1;

FIG. 7 is a side view of a prior art screw and a top view of a prior artspacer with an elongated hole;

FIG. 8 is a side view of a screw and a top view of a spacer of theelectron gun of FIG. 1;

FIG. 9 is a graph of a power density distribution of an electron beamresulting from a prior art thermionic electron source; and

FIG. 10 is a graph of a power density distribution of an electron beamresulting from the thermionic electron source of FIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is disclosed herein with respect to a best modeembodiment for use in an electron gun illustrated in FIG. 1. Referringnow to FIG. 1, an electron gun 20 for an electron beam furnace (notshown) has an accelerating electrode 22, a platform 24, and a head 26.The electron gun 20 has a power rating of about sixty five kilowatts.The electron gun 20 is representative of in shape, but larger, thanelectron guns (not shown) having a power rating less than sixty fivekilowatts, e.g., forty-five kilowatts. The accelerating electrode 22 ofthe electron gun 20 has a plate portion 28 and a beam shaper portion 30.The beam shaper portion 30 is elongated and extends in a longitudinaldirection L. The accelerating electrode 22 further has an elongatedaperture 32 that extends in the longitudinal direction L and provides apath for electrons, generated by the head 26, to exit the electron gun20. The accelerating electrode 22 may comprise a copper material and maybe formed as one piece for example by milling. Alternatively, theaccelerating electrode 22 may be an assembly wherein the plate portion28 comprises a stainless steel material and the beam shaper portion 30comprises a copper material.

A plurality of bolts 34 connects the accelerating electrode 22 to acooling plate 36 and a deflection system 38. Washer plates 40 recessedin the accelerating electrode 22 help distribute the load applied by thebolts 34. The cooling plate 36 is part of a cooling system, representedin part by a pair of water pipes 42. The cooling system 42 and thedeflection system 38 are specifically designed to suit the physical sizeof the accelerating electrode 22 and the power rating of the electrongun 20. For example, the cooling plate 36 is sized to contact as much ofthe surface area of the accelerating electrode 22 as is possible withoutcreating interference to other structures on the accelerating electrode22. This maximizes heat transfer between the accelerating electrode 22and the cooling system 42 and thereby helps prevent the electron gun 20from overheating. The cooling plate 36 and the deflection system 38 areillustrative of but physically larger than a cooling plate and adeflection system respectively suited to the physical size and the powerrating of an electron gun having a power rating of forty five kilowatts.Consequently, the cooling plate 36 and the deflection system 38 are toolarge to connect to the accelerating electrode of an electron gun havinga power rating of forty five kilowatts. The accelerating electrode 22may be electrically grounded by way of its connection to the coolingplate 36 and the deflection system 38.

The accelerating electrode 22 supports a first platform support 44 and asecond platform support 46. The first platform support 44 comprises ahigh voltage insulator 50 and an insulator cover 52. The high voltageinsulator 50, which may comprise a ceramic material, has a first end 54and a second end 56. The first end 54 has a threaded stud 58 thatextends through a washer 60 and the accelerating electrode 22 andengages a nut 62 to retain the high voltage insulator 50 to theaccelerating electrode 22. The second end 56 of the high voltageinsulator 50 has a shoulder 64 and a threaded stud 66. The shoulder 64abuts a collar 68 on the insulator cover 52. The threaded stud 66extends through the insulator cover 52 and engages a threaded cap 70 toretain the high voltage insulator 50 to the insulator cover 52. Theinsulator cover 52 limits formation of deposits on the high voltageinsulator 50. The insulator cover 52 further includes a threadedengagement surface 71. The second platform support 46 comprises a highvoltage insulator 72, an insulator cover 73, and an insulator cover 74,which are identical to the high voltage insulator 50, the insulatorcover 52, and the threaded cap 70 of the first platform support 44,respectively.

The first platform support 44 and the second platform support 46 eachfurther comprise a support ring, represented by a support ring 75. Thesupport rings 75 each have a threaded engagement surface (not shown) anda support surface, represented by a support surface 76. The threadedengagement surfaces of the support rings 75 engage the threadedengagement surfaces of the insulator covers 52, 73 to retain the supportrings 75 to the insulator covers 52, 73. The support surfaces 76 of thesupport rings 75 provide support for the platform 24 and space theplatform 24 apart from the accelerating electrode 22. Retaining rings 78engage the insulator covers 52, 73 and retain the platform 24 on thesupport surfaces 76 of the support rings 75. Adjusting the spacingbetween one of the support surfaces 76 and the accelerating electrode 22is accomplished by loosening one of the support ring 75 and theretaining ring 78, and subsequently tightening the other of the supportring 75 and the retaining ring 78. Adjusting the spacing between one ormore of the support surfaces 76 and the accelerating electrode 22 ineffect adjusts the spacing between the platform 24 and the acceleratingelectrode 22.

The platform 24, which supports the head 26 of the electron gun 40, hasan opening 80. The platform 24 further has a first locating member 82and a second locating member 84 disposed on opposite sides of theopening 80. The first locating member 82 comprises a spacer 86 and aprojection 88 extending therefrom. The second locating member 84comprises a spacer 90 and a projection 92 extending therefrom. Each ofthe projections may have a notch, represented by a notch 94, which arepart of a detent mechanism described hereinbelow. The spacer 86 of thefirst locating member 82 and the spacer 90 of the second locating member84 each have two holes, represented by a hole 96. Screws 98 extendthrough the holes 96 and engage the platform 24 to retain the firstlocating member 82 and the second locating member 84 to the platform 24.The screws 98 each have a head and a shank, represented by a shank 100.The shank 100 has a diameter 102. The holes 96 are preferably elongatedrelative to the diameter 102 of the shank 100 in a widthwise directionW, transverse to the longitudinal direction L, to provide a clearancebetween the spacers 86, 90 and the shank 100 of the screws 98. Thisclearance facilitates adjustment of the position of the locating memberrelative to the platform 24 and the accelerating electrode 22. Ifdesired, the holes 96 may be elongated in more than one directionrelative to the shank diameter 102. However, elongating the holes inonly one direction relative to the shank diameter helps to preventmisalignment between a thermionic electron source, describedhereinbelow, and the elongated aperture 32 of the accelerating electrode22. Repositioning of the first locating member 82 or the second locatingmember 84 is accomplished by loosening one or both of the screws 98 thatretain the locating member to the platform 24, positioning the locatingmember, and subsequently re-tightening the screws 98.

Referring now to FIG. 7, a prior art locating member 104A has a spacer104B with a hole 104C that is elongated. The hole 104C has a dimension104D of less than ten mm in the widthwise direction W. The hole 104C hasa dimension 104F of about 6.5 mm in the longitudinal direction L. Ascrew 104H employed in connection with the hole 104C to position thelocating member 104A has a head and a shank. The shank is threaded andhas a diameter 104I of five mm. The dimensions of the screw 104H and thehole 104C result in clearance between the spacer 104B and the screw 104Hand thereby result in adjustability of the locating member 104A. Theadjustability in the widthwise direction W is less than five mm (lessthan 10 mm−5 mm). The adjustability in the longitudinal direction L isabout 1.5 mm (about 6.5 mm−5 mm). The adjustability in the widthwisedirection W and the adjustability in the lengthwise direction L areintended to compensate for manufacturing tolerances of components of theprior art electron gun (not shown) and to facilitate alignment of theelectron source and the accelerating electrode in the prior art electrongun. Consequently, as will be understood in view of the discussionhereinbelow, there is less than desired adjustability with the prior artlocating member 104A to position a head from an electron gun that has apower rating of forty five kilowatts, which is physically smaller thanthe head 26 of the electron gun 20.

Referring now to FIG. 8, in one embodiment of the present invention, thehole 96 has a dimension 106 at least fifteen mm in the widthwisedirection W. The hole 96 has a dimension 108 of six mm in the lengthwisedirection L. The shank 100 of the screw 98 has a portion with a collarand a portion that is threaded. The portion with the collar has adiameter 110 of 5.95 mm and a dimension 111 that is less than thethickness of the spacer 90. The portion that is threaded has a dimension109 of 5 mm to engage the platform 24 (FIG. 1). The dimensions of thescrew 98 and the hole 96 result in clearance between the spacer 90 andthe screw 98 and thereby adjustability of the locating member 84. Theadjustability in the widthwise direction W is at least nine mm ( 15 mm−6mm). The adjustability in the lengthwise direction L is 0.05 mm (6mm−5.95 mm). As will be evident in view of the discussion hereinbelow,the adjustability in this embodiment is enough to position the head 26of the electron gun 20, and enough to position a head from an electrongun having a power rating of forty five kilowatts.

Referring again to FIG. 1, the head 26 includes a frame member 112 thatis U-shaped and comprises a stainless steel material. The frame member112 has a first side wall 113 and a second side wall 114. The first sidewall 113 has a first reference member 116 having the shape of a recess.The second side wall 114 has a second reference member 118 having theshape of a recess. The first reference member 116 and the secondreference member 118 have a distance D between them. The distance Ddepends on the size of the head 26, which in turn depends on the powerrating of the electron gun 20. For the sixty-five kilowatt electron gun20, the distance D is ffly mm, center to center. Note that for aforty-five kilowatt electron gun, the distance between reference membersis forty-five mm, which is five mm (50 mm−45 mm) less than that of theelectron gun 20. When the frame member 112 is placed on the platform 24,the first reference member 116 and the second reference member 118engage the first locating member 82 of the platform 24 and the secondlocating member 84 of the platform 24, respectively, to position thehead 26 on the platform 24 and thereby positioning the head 26 in threedimensions relative to the accelerating electrode 22.

The frame member 112 may further have a pair of catch assemblies,represented by a catch assembly 122. Each catch assembly 122 has a ball124 and a spring 126. The catch assemblies 122 cooperate with thenotches 94 in the projections 88, 92 to define a detent mechanism thatretains the head 26 to the platform 24. For example, the ball 124engages the notch 94 in the projection 92 of the second locating member84. The spring 126 biases the ball 124 toward the notch 94. A pair ofscrews, represented by a screw 128, adjusts the bias provided by thesprings 126.

The head 26 further includes a first terminal 130, a second terminal132, and a thermionic cathode assembly 134. The first terminal 130engages the frame member 112. The second terminal 132 engages aconductor 136. The conductor 136 mechanically and electrically connectsthe second terminal 132 to a pair of non-magnetic, spring conductors 138that extend through the opening 80 of the platform 24 and support thethermionic cathode assembly 134. A plurality of screws 140 connects theconductor 136 to an insulator 142. Wedge shaped members 144 clamp theinsulator 142 to the frame member 112. A plurality of screws 146 biasesthe wedge shaped members 144 toward the insulator 142.

The thermionic cathode assembly 134 includes a thermionic electronsource 150 and a focusing electrode 152. The thermionic electron source150 and the focusing electrode 152 are spaced apart from one another andeach extends in the longitudinal direction L. The thermionic electronsource 150 is one piece and may comprise a tungsten material. Thefocusing electrode 152 extends partially around the thermionic electronsource 150 along a portion of a length 154 of the thermionic electronsource 150. The focusing electrode 152 has a notch 156 that extends inthe longitudinal direction L. The notch 156 is bordered by a recessedsurface 158. An ion trap 160 extends longitudinally and into the notch156 so as to be between the recessed surface 158 of the focusingelectrode 152 and the thermionic electron source 150. The ion trap 160is sacrificial in that it is expected that the ion trap 160 will bebombarded by ions and erode over time. The ion trap 160 reduces theamount of bombardment and erosion experienced by the focusing electrode152. The ion trap 160 is less costly to replace than the focusingelectrode and may comprise a carbon material. Note that the opening 80of the platform 24 is large enough for the thermionic cathode assembly134 to pass through so as to facilitate positioning the thermioniccathode assembly 134 proximate to the accelerating electrode 22.

The head 26 further includes a first holder 162 and a second holder 164.The first holder 162 is mechanically and electrically connected to thefocusing electrode 152 by fasteners 166. The first holder 162 has aclamping plate 168 and a screw 170. The screw 170 engages the clampingplate 168 and thereby causes it to tightly engage the thermionicelectron source 150. The second holder 164 is mechanically andelectrically connected to the frame member 112 by fasteners 172. Thesecond holder 164 has a clamping plate 174 and a screw 176. The screw176 engages the clamping plate 174 and thereby causes it to tightlyengage the thermionic electron source 150.

Referring now to FIGS. 2-5, the thermionic electron source has a firstend portion 180, a second end portion 182, and an aperture disposedtherebetween 184. The aperture 184 extends a portion of the length 154and a portion of a width 186 of the thermionic electron source 150. Thethermionic electron source 150 further comprises a first longitudinalportion 190 and a second longitudinal portion 192 that extend in thelongitudinal direction L and are spaced apart from one another by theaperture 184. The first end portion 180 and the second end portion 182rigidly join the first longitudinal portion 190 and the secondlongitudinal portion 192 together. The thermionic electron source may begenerally uniform in thickness 194. As illustrated, the aperture 184diminishes in width (i.e., tapers) near the ends of the firstlongitudinal portion 190 and the second longitudinal portion 192,although the aperture is not limited to such. The tapering helps toreduce buildup of stress and aids fabrication of the thermionic electronsource.

Referring also now to FIG. 6, the first longitudinal portion 190 has asurface 196 that opposes the accelerating electrode 22; the secondlongitudinal portion 192 has a surface 198 that opposes the acceleratingelectrode 22. The surface 196 and the surface 198 may be inclined andface toward each other. Due to the incline, the thermionic electronsource 150 may have a widthwise cross section having the shape of achevron, as illustrated in FIG. 4. The incline of the surfaces 196, 198may diminish near the ends of the longitudinal portions in an effort tominimize stress. Making the surfaces 196, 198 inclined rather thancoplanar with each other is a way to increase to the width of theaperture without decreasing the surface area of the surfaces 196, 198.Note that the power rating of the electron gun 20 is related to thesurface area of the surfaces that face toward the accelerating electrode22. Depending on the incline and the width of the aperture 184, thethermionic electron source 150 may have almost as much surface areafacing toward the accelerating electrode 22 as the thermionic electronsource 20 would have in the absence of the aperture 184. The thermionicelectron source 150 and the ion trap 160 are preferably aligned with theelongated aperture 32 of the accelerating electrode 22, to maximize thebenefit of the aperture and the ion trap described below.

The thermionic electron source 150 may be fabricated using any suitablemethod including but not limited to pressing, rolling, and machining(including but not limited to electrical discharge machining and lasermachining) and combinations thereof. The thermionic electron source 150may be fabricated from a thermionic electron source that does not havean aperture 184 and has been used in the electron gun 20 and undergoneion bombardment.

For the electron gun 20, which has a power rating sixty five kilowatts,the thermionic electron source 150 has a length 154 of one hundred mmand a width 186 of about three mm. The focusing electrode 152 (FIGS. 1,6) has a length of sixty-five mm and a width of about thirty-two mm.There is clearance 200 of about 0.5 millimeter between the thermionicelectron source 150 and the focusing electrode 152. The sacrificial iontrap 160 (FIGS. 1, 6) has a length equal to that of the focusingelectrode 152 and has a width in a range of about 1.5 mm to about twomm. The length of the aperture 184 is about sixty mm, which is about tenpercent less than the sixty-five millimeter length of the focusingelectrode 152. The width of the aperture 184 is about 0.75 mm, which isabout one-quarter of the width 186 of the thermionic electron source150. Note that the length 154 of the thermionic electron source 150 andthe length of the focusing electrode 152 typically depend on the powerrating of the electron gun, but the width 186 of the thermionic electronsource and the width of the focusing electrode 152 typical do not dependon the power rating of the electron gun. For example, for an electrongun having a power rating of forty-five kilowatts, the thermionicelectron source 150 has a length 154 of eighty mm and a width 186 ofabout three mm. The focusing electrode 152 has a length of forty five mmand a width of about thirty two mm.

In operation, the first terminal 130 and the second terminal 132 areconnected to a power supply (not shown). The power supply provides asource of electric current for the thermionic electron source 150. Theelectric current flows through the first terminal 130, the second holder164, the thermionic electron source 150, the first holder 162, thefocusing electrode 152, the spring conductors 138, the conductor 136,and the second terminal 132. As the electric current flows through thethermionic electron source 150 it results in heating thereof, to arelatively high temperature, but typically below the melting temperatureof tungsten, causing the thermionic electron source 150 to emitelectrons. The voltage across the thermionic electron source 150 istypically less than ten volts. Because the heating for the thermionicelectron source results from an electric current, the electron gun isreferred to as directly heated. A second power (not shown) provides thesecond terminal 132 with a negative voltage potential (typically about−20 kilovolts), which is in turn provided to the first holder 162 andthe focusing electrode 152 through the conductor 136 and the springconductor 138. The negative voltage potential causes the focusingelectrode 152 to repel the electrons and thereby direct the electrons ina direction generally toward the accelerating electrode 22. Theaccelerating electrode 22 is typically at an electrical ground voltagepotential by way of the connection between the accelerating electrode22, the cooling system 42, and the deflection system 38. Theaccelerating electrode causes the electrons to form into a beam andtravel in the downstream direction. The electron beam exits the electrongun 20 through the elongated aperture 32 of the accelerating electrode22.

The electron beam from the electron gun typically extends in thelongitudinal direction L and the widthwise direction W, and has agenerally rectangular cross section in a plane containing thelongitudinal direction L and the widthwise direction W. The electronbeam has a power density that varies across its width (i.e., in thewidthwise direction W). Referring now to FIG. 9, a graph illustrates apower density distribution obtained from the electron gun with a priorart thermionic electron source. The power density distribution hascharacteristics similar to that of a Gaussian distribution. Referringnow to FIG. 10, a graph illustrates a power density distributionobtained from the electron gun 20 with the thermionic electron source150. The power density distribution has characteristics similar to thatof a Gaussian distribution, but with some variation due to the aperture184 of the thermionic electron source.

The shape and the power density distribution of the electron beamdepends on he distance between the thermionic electron source 150 andthe accelerating electrode 22, and also depends on the differencebetween the voltage potential of the thermionic electron source 150 andthe voltage potential of the accelerating electrode 22. However, becausethe platform 24 is adjustably spaced from the accelerating electrode 22,various electron beam shapes and various power density distributions maybe obtained by moving the platform 24 closer to or farther away from theaccelerating electrode 22, without the need to vary the voltagepotential between the thermionic electron source and the acceleratingelectrode. There is preferably at least one inch of adjustability tomake possible a wide range of electron beam shapes and power densitydistributions.

The electron beam is used in the electron beam furnace (not shown) tovaporize materials for deposition on articles. Positively charged ionsare produced as a result of the vaporization and of the material in theelectron beam furnace. Some of these ions have a direction of travelopposite that of the electrons in the electron beam causing the ions totravel through the elongated aperture in the accelerating electrode andtoward the thermionic electron source. The ions have the potential tobombard and erode the thermionic electron source.

However, because the thermionic electron source has an aperture, many ofthese positively charged ions do not strike the thermionic electronsource 150, but rather pass through the aperture and strike thesacrificial ion trap. The thermionic electron source 150 is thus lesssusceptible to ion bombardment and thus has a longer operating life thanprevious ribbon type thermionic electron sources. The life expectancy ofthe thermionic electron source depends on the operating conditions,however, for a given set of operating conditions, the life expectancy ofthe thermionic electron source is about two times greater than it wouldbe without the aperture 184. Moreover, because the improved electronsource is one piece, use of the electron source does not require supportand relative positioning of multiple emitters such as that required byan electron source having two separate and parallel emitters. Inaddition, the one piece construction may make the electron source morerigid and thus more durable and less likely to deform than an electronsource having two separate emitters.

As stated above, it is important that an electron gun to be used in anelectron beam furnace generate an electron beam suitable for the type ofmaterial to be heated and the type of deposition sought for the article.Different types of materials and depositions require electron beams ofdifferent amounts of electron beam power and may require differentelectron beam shape. It is also desirable to have the electron beam gunoperate in a space charge limited mode. However, the sixty five kilowattelectron gun operates in space charge limited mode within a power rangefrom about forty three kilowatts to about seventy kilowatts.Consequently, the sixty five kilowatt electron gun cannot adequatelygenerate electron beams for all of the electron beam powers required.

It has been determined that the accelerating electrode 22 of theelectron gun 20 can be operated with the head 26 of the electron gun 20or alternatively with the head of a second electron gun having a powerrating substantially less than the sixty five kilowatt power rating ofthe electron gun 20. As used herein substantially less than means atleast twenty five percent less than. In this alternative, the electronbeam that results is comparable to that which would be generated by thesecond electron gun. For example, an electron gun comprising theaccelerating electrode 22 of the sixty five kilowatt gun and the head ofa forty five kilowatt gun generates an electron beam comparable to thatgenerated by the forty five kilowatt electron gun. A forty five kilowattelectron gun operates in space charge limited mode within a power rangefrom about twenty seven kilowatts to about forty eight kilowatts. Theaccelerating electrode 22 of the electron gun 20 and the head of a fortyfive kilowatt electron gun operate together in space charge limited modewithin the same range as that of the forty five kilowatt electron gun.Since the accelerating electrode 22 is not replaced and the resultingelectron gun operates at a power less than the power rating of theelectron gun 20 (sixty five kilowatts), there is no need to replace thedeflection system 38 or the cooling system 42. Therefore, suitableelectron beams of various power levels can be generated withoutreplacing the accelerating electrode 22 of the electron gun 20, thedeflection system 38 or the cooling system 42, thereby reducing theeffort involved.

Providing locating members 82, 84 that are adjustably located by atleast nine mm facilitates the operation of the accelerating electrode 22of the electron gun 20 with the head of a second electron gun that maybe smaller in size than the head 26 of the sixty five kilowatt electrongun 20. The distance between the first locating member and the secondlocating member 84 can be sufficiently varied so as to correspond to thedistance D between the reference members of the head of the secondelectron gun. For example, as stated above, for a forty-five kilowattelectron gun, the distance between reference members on the head isforty-five mm, which is five mm (50 mm−45 mm) smaller than that of theelectron gun 20. In contrast, the prior art locating member 104A hasadjustability of less than five mm and is intended to compensate formanufacturing tolerances of components of the prior art electron gun andto facilitate alignment of the electron source and the acceleratingelectrode in the prior art electron gun. Consequently, there is lessthan desired adjustability with the prior art locating member 104A toposition a head that is five mm smaller than the head 26 of the electrongun 20.

In regard to the thermionic electron source 150, the opposite endportions 180, 182 need not be the same as each other, and thelongitudinal portions 190, 192 need not be the same as each other. Itshould be understood that the adjustable locating members 82, 84 and theadjustable platform supports 44, 46 are not required. Nor is itnecessary to be able to replace the head 26 of the electron gun 20.Although shown with one focusing electrode 152 and one acceleratingelectrode 22, there may be any number of focusing and acceleratingelectrodes. A sacrificial ion trap 160 need not be employed.Furthermore, the present invention may be used with electron guns of anypower rating.

While the present invention has been described with reference to a bestmode embodiment, this description is not meant to be construed in alimiting sense. Various modifications of the best mode of embodiment, aswell as additional embodiments of the invention, will be apparent topersons skilled in the art upon reference to this description, withoutdeparting from the spirit of the invention, as recited in the claimsappended hereto. It suffices for the broadest scope of the inventionthat an apparatus for an electron gun include a one piece thermionicelectron source that extends in a longitudinal direction and hasopposite end portions, an aperture disposed therebetween, and twolongitudinal portions that extend in the longitudinal direction, arespaced apart from one another by the aperture and rigidly joinedtogether by the opposite end portions. It is therefore contemplated thatthe appended claims will cover any such modifications or embodiments asfall within the true scope of the invention.

What is claimed is:
 1. An apparatus comprising a one piece thermionicelectron source that extends in a longitudinal direction and hasopposite end portions, an aperture disposed therebetween, and twolongitudinal portions that extend in the longitudinal direction, arespaced apart from one another by the aperture and rigidly joinedtogether by the opposite end portions; a focusing electrode extendingpartially around the one piece thermionic electron source and along atleast a portion of the length of the one piece thermionic electronsource, the focusing electrode having a notch that extends in thelongitudinal direction; and a sacrificial ion trap extendinglongitudinally and disposed between a surface of the focusing electrodeand the one piece thermionic electron source.
 2. The apparatus of claim1 wherein each of the longitudinal portions has a surface that isinclined toward that of the other.
 3. An apparatus comprising a onepiece thermionic electron source of generally uniform thickness thatextends in a longitudinal direction and has opposite end portions, anaperture disposed therebetween, and two longitudinal portions thatextend in the longitudinal direction, are spaced apart from one anotherby the aperture and rigidly joined together by the opposite endportions; a focusing electrode extending partially around the one piecethermionic electron source and along at least a portion of the length ofthe one piece thermionic electron source, the focusing electrode havinga notch that extends in the longitudinal direction; and a sacrificialion trap extending longitudinally and disposed between a surface of thefocusing electrode and the one piece thermionic electron source.
 4. Anapparatus comprising a one piece thermionic electron source that extendsin a longitudinal direction and has opposite end portions, an aperturedisposed therebetween, and two longitudinal portions that extend in thelongitudinal direction, each of the two longitudinal portions having asurface that is inclined toward each other, are spaced apart from oneanother by the aperture and rigidly joined together by the opposite endportions; a one piece thermionic electron source that extends in alongitudinal direction and has opposite end portions, an aperturedisposed therebetween, and two longitudinal portions that extend in thelongitudinal direction, are spaced apart from one another by theaperture and rigidly joined together by the opposite end portions; and asacrificial ion trap extending longitudinally and disposed between asurface of the focusing electrode and the one piece thermionic electronsource.
 5. An apparatus comprising a one piece thermionic electronsource that extends in a longitudinal direction and has opposite endportions, an aperture disposed therebetween, and two longitudinalportions that extend in the longitudinal direction, are spaced apartfrom one another by the aperture and rigidly joined together by theopposite end portions, wherein the aperture diminishes in width nearends of the longitudinal portions; a one piece thermionic electronsource that extends in a longitudinal direction and has opposite endportions, an aperture disposed therebetween, and two longitudinalportions that extend in the longitudinal direction, are spaced apartfrom one another by the aperture and rigidly joined together by theopposite end portions; and a sacrificial ion trap extendinglongitudinally and disposed between a surface of the focusing electrodeand the one piece thermionic electron source.
 6. An apparatus comprisinga one piece thermionic electron source of generally uniform thicknessthat extends in a longitudinal direction and has opposite end portions,an aperture disposed therebetween, and two longitudinal portions thatextend in the longitudinal direction, each of the two longitudinalportions having a surface that is inclined toward each other, are spacedapart from one another by the aperture and rigidly joined together bythe opposite end portions; a one piece thermionic electron source thatextends in a longitudinal direction and has opposite end portions, anaperture disposed therebetween, and two longitudinal portions thatextend in the longitudinal direction, are spaced apart from one anotherby the aperture and rigidly joined together by the opposite endportions; and a sacrificial ion trap extending longitudinally anddisposed between a surface of the focusing electrode and the one piecethermionic electron source.
 7. An apparatus comprising a one piecethermionic electron source of generally uniform thickness that extendsin a longitudinal direction and has opposite end portions, an aperturedisposed therebetween, and two longitudinal portions that extend in thelongitudinal direction, are spaced apart from one another by theaperture and rigidly joined together by the opposite end portions,wherein the aperture diminished in width near ends of the longitudinalportions; a focusing electrode extending partially around the one piecethermionic electron source and along at least a portion of the length ofthe one piece thermionic electron source, the focusing electrode havinga notch that extends in the longitudinal direction; and a sacrificialion trap extending longitudinally and disposed between a surface of thefocusing electrode and the one piece thermionic electron source.
 8. Anapparatus comprising a one piece thermionic electron source that extendsin a longitudinal direction and has opposite end portions, an aperturedisposed therebetween, and two longitudinal portions that extend in thelongitudinal direction, each of the two longitudinal portions having asurface that is inclined toward each other, are spaced apart from oneanother by the aperture and rigidly joined together by the opposite endportions, wherein the aperture diminishes in width near ends of thelongitudinal portions; a focusing electrode extending partially aroundthe one piece thermionic electron source and along at least a portion ofthe length of the one piece thermionic electron source, the focusingelectrode having a notch that extends in the longitudinal direction; anda sacrificial ion trap extending longitudinally and disposed between asurface of the focusing electrode and the one piece thermionic electronsource.
 9. An apparatus comprising a one piece thermionic electronsource of generally uniform thickness that extends in a longitudinaldirection and has opposite end portions, an aperture disposedtherebetween, and two longitudinal portions that extend in thelongitudinal direction, each of the two longitudinal portions having asurface that is inclined toward each other, are spaced apart from oneanother by the aperture and rigidly joined together by the opposite endportions, wherein the aperture diminishes in width near ends of thelongitudinal portions; a focusing electrode extending partially aroundthe one piece thermionic electron source and along at least a portion ofthe length of the one piece thermionic electron source, the focusingelectrode having a notch that extends in the longitudinal direction; anda sacrificial ion trap extending longitudinally and disposed between asurface of the focusing electrode and the one piece thermionic electronsource.
 10. The apparatus of claim 5 further comprising an acceleratingelectrode positioned apart from the one piece thermionic electronsource.
 11. The apparatus of claim 8 further comprising an acceleratingelectrode positioned apart from the one piece thermionic electron sourceand the focusing electrode.
 12. The apparatus of claim 9 furthercomprising an accelerating electrode positioned apart from the one piecethermionic electron source and the focusing electrode.